How to Design a Scissor Truss

A roof truss carries the weight of the roof structure and prevents the building from flexing and bending, particularly in homes with two or more levels. The triangular shape of the truss helps distribute pressure throughout the entire component, which makes it rigid and stable. Homebuilders install scissor trusses when the home design requires a unique slope known as a “cathedral ceiling.” Scissors trusses attach to two wall sections. The builder usually spaced trusses 24 inches on center.

Things You'll Need

Structural Engineer

2-by-6 inch lumber, structural grade (No.1 or No.2)

Miter saw

Galvanized screws, 1.5-inch long

2-by-4 inch lumber, stud grade (No. 3)

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Hire a structural engineer to help design the scissor truss. Incorporate local building code requirements into the design, particularly in regard to roof construction, such as sheathing, insulation and covering. It must also take into account the span, which is the distance between the wall section and average amount of snowfall. The structural engineer has the skills to calculate the proper strength of the component, which is essential to prevent the roof from twisting or collapsing.

Purchase high-grade lumber to create the chords for the scissor truss. The chords refer to the components that form the triangle of the truss, including the outer components of the bottom chord. This portion of the scissor truss carries the most weigh from the structure. Select 2-by-6 or 2-by-8 inch, structural grade, No.1 or No.2 lumber. Use No. 3 stud-grade lumber to form the interior components, which makes the “web” of the truss. Make sure the lumber is straight. Alternatively, face the curve down to flatten the lumber once the structural weight is applied.

Measure and cut 2-by-6 inch lumber to form the chords for one truss. Use a miter saw to make the clean, angled cuts necessary for an accurate fit of the components. Obtain the measurements from the design completed by the structural engineer. Join the chords together to ensure the dimensions fit tightly together.

Secure the chords together -- at the joints on both sides -- with galvanized steel gusset plates. Use 1.5-inch long galvanized screws to attach the gusset plates. Install a 2-by-4 inch brace from the bottom chord to the center-top of the truss and at other angles. Base the length and angle of bracing components on the engineer’s design. Use gusset plates to secure the joints. Move and carry trusses in a vertical position, which alleviates stress on the joints.